#include<vector>
using namespace std;
struct ListNode {
      int val;
      ListNode *next;
      ListNode(int x) : val(x), next(NULL) {}
 };

class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        if(head==NULL || head->next==NULL){
            return head;
        }

        ListNode *node = reverseList(head->next);
        head->next->next = head;
        head->next = NULL;
        return node;
    }


    ListNode* reverseList2(ListNode* head) {
        if(head==NULL || head->next==NULL){
            return head;
        }

       ListNode *pre=NULL;
       ListNode*curr=head;
       while (curr!=NULL){
           ListNode*next = curr->next;
           curr->next = pre;
           pre = curr;
           curr = next;
       }
       return pre;    
    }
};